This invention relates to novel image recording materials, and more particularly to a novel image recording material on which a continuous tone original is contact-printed or projection-printed so that an image with gradation is obtained through development or simple after-treatment.
Silver-salt photosensitive materials to record toned or gradational images are known in the art. Furthermore, gradational images can be recorded with non-silver-salt photosensitive materials such as free radical photosensitive materials, photochromic photosensitive materials, heat-sensitive materials, and diazo photosensitive materials.
The mechanism of recording gradational images by using the above-described photosensitive materials will be briefly described. The material of the photosensitive layer reacts in accordance with the exposure energy, and differences between the quantities of substances produced by the reaction are converted into differences in optical density to record a gradational image. Alternatively, after-treatment is effected with substances produced by exposure as nuclei of development, as a result of which substances causing optical density differences are produced to record a gradational image.
The above-described photosensitive materials can satisfactorily record continuous tone images. However, unlike the image recording material of the invention, they cannot be used as printing forms, that is, the images of originals recorded thereon cannot be used as printing forms. Besides the methods of using the above-described photosensitive materials, the following methods are employed in the fields of printing and makeup to provide gradational images.
(1) A method in which a contact screen is used.
A continuous tone original is printed on a lith type film or the like through a halftone film called "a contact screen", so that a continuous tone image based on the optical density differences of an original is converted into minute dots and tone reproduction is effected by utilizing the differences in size of the dots.
If the image formed by the dots is printed on a PS plate, then a printing form can be obtained. In multi-color printing, the colors of an original are separated into yellow, magenta, cyan and black, and printing forms are prepared respectively for these colors in the above-described method. The images of the original different in color are printed repeatedly on one and the same sheet to provide a colored image. In this case, a problem arises in that a moire pattern due to the regular mesh of the contact screen appears. Furthermore, the method suffers from an operational difficulty accompanying the use of the contact screen.
(2) A method in which a screenless lithography is used.
A film of diazonium salt photosensitive material is formed on a grained aluminum plate, and the film thus formed is exposed to light, only through a continuous tone original, thereby to provide a lithographic printing plate. This method may be practiced in two ways depending on the kinds of diazonium salts. In one method, positive-to-positive conversion is carried out, and in the other method, negative-to-positive conversion is carried out. There are two kinds of diazonium salts usable in the method. In one of the two kinds of diazonium salts, the hydrophobic characteristic thereof is changed to the hydrophilic characteristic by optical decomposition, and in the other the hydrophilic characteristic is changed to the hydrophobic characteristic. The former is used for the positive-to-positive conversion type screenless lithography, and the latter is used for the negative-to-positive conversion type screenless lithography. The decomposition reaction of the diazonium salt coated on the aluminum plate is advanced to a depth corresponding to the quantity of exposure. When a positive type diazonium salt is used, planar regions of the coated film wherein the entire thickness has not been changed from the hydrophobic characteristic to the hydrophilic characteristic, remain on the aluminum plate even after development by water washing, thereby to effect the positive-to-positive image conversion. On the other hand, when a negative type diazonium salt is used, planar regions of the coated film wherein the entire thickness has not been changed from the hydrophilic characteristic to the hydrophobic characteristic are removed from the aluminum plate through development by water washing.
The mechanism of obtaining a gradational printing form from a continuous tone original according to the method will be briefly described with reference to the positive-to-positive conversion type screenless lithography. The thickness of the film of diazonium salt coated on a grained aluminum plate is variable in correspondence to the grained surface of the aluminum plate. In the regions of the diazonium salt film subjected to a relatively large exposure energy depending on the tone of the original, the diazonium salt is decomposed to the bottom of the grain. Accordingly, a hydrophilic aluminum surface appears upon development, and this surface repels ink in printing. If, in contrast, the exposure energy is less, then the decomposition is less, as a result of which a lipophilic diazonium salt is left on the aluminum plate, and the lipophilic diazonium salt thus left attracts ink in printing. In the regions of the diazonium salt film where the exposure energy is between those in the two cases described above, the decomposition is caused to half of the depth of the grain, and accordingly the ink adhering area is between those in the two cases described above.
Thus, the ink adhering area is varied in accordance with the configuration of the grains formed on the surface of the aluminum plate and also the exposure energy, and therefore the tone of the original is reproduced.
The method is advantageous in that it is unnecessary to use a contact screen. However, it is still not fully satisfactory in that it is difficult to provide a printing form stable in quality because the tone reproduction of printed images is changed by delicate variations in quantity of coating diazonium salt and by variations in configuration of grains.
(3) A method in which collotype printing is employed.
In this method, a continuous tone image is printed by utilizing the optical curing of a dichromate gelatin film formed on a glass plate.
The mechanism will be briefly described. The gelatin in the regions of the dichromate gelatin film subjected to a relatively large exposure energy, is cured. That is, the gelatin no longer absorbs cold water but readily attracts fatty ink. In contrast, the gelatin in the regions of the film which has not been exposed to light absorbs cold water well but does not attract fatty ink. The gelatin in the regions of the film where the exposure energy is between those in the above-described two cases is changed so that the water absorption characteristic and the fatty ink adhering characteristic are varied in proportion to the exposure energy, as a result of which the middle tone parts of the original can be printed. The method is advantageous in that it is unnecessary to use a contact screen and no moire pattern is formed in multi-color printing. However, since the surface of the printing form is of the gelatin film, its mechanical strength is weak, and therefore it is difficult to print a number of copies with one printing plate formed in accordance with the method. In addition, it is difficult for the method to provide images which can be readily observed with the naked eye, by itself or without the printing step.